Combined cycle electric power generation uses a gas turbine and a steam turbine. In production facilities, electric energy is currently based primarily on the use of hydroelectric, nuclear and fossil large-scale electricity supply systems via transmission and distribution. Reduction targets of CO2 emissions set in many countries, will bring an increase in the share of renewable energy that will reach, by 2020, 35% in Germany and 20% in France, for example. In addition to increasing their efficiency, conventional energy sources will increase their flexibility and responsiveness to be used more optimally to integrate a wider variety of intermittent power sources, such as wind farms, solar and others such as tidal power, geothermal or biomass-fed. This diversification and increasing the number of sources of production represents and involves a considerable challenge for management systems and distribution of electricity networks.
The means of generation of electrical energy and current distribution networks (Grid) were not designed to be subjected to these changes, and are thus inadequate to meet these new requirements in the long term without substantial investment. This level of renewable energy production enhances the complexity of systems and electricity distribution networks, resulting in energy fluctuations, which must be carefully controlled. Without careful control, distribution systems may operate inefficiently or may be subjected to frequent disturbances.
From the perspective of electricity suppliers and operators of distribution, the solutions must involve:                Increasing the flexibility of conventional power plants;        The introduction of energy storage technologies for use at any level, to cushion peak demands and allow the integration of more renewable energy sources;        The introduction of more flexible delivery methods to cope with fluctuations in supply, increase efficiency and optimize the system operation;        The introduction of systems for forecasting, monitoring and controlling power to prevent any disturbances.        
Disturbances caused by the announced change of electrical power sources and distribution systems can cause power outages (blackouts). That is, a loss in the short or long term of electrical energy over an area due to defects in the distribution system (electrical transmission lines or sub-stations) can cause a short-circuit or overload of the main system. In particular, a “Blackout” or loss of AC power is critical to public safety, hospitals, factories wastewater treatment, mining, etc. Other critical systems, such as telecommunications systems, are also required to have a source of emergency power. That is why the systems are equipped with backup generators that start automatically when power is lost.
The appearance of defects in a Grid near a central power generation type gas turbine, steam turbine or combined cycle plant can also create disturbances or even cause the shutdown of the plant. In addition, a central electricity generation draws energy to the network to initiate the increase in speed of the turbine using an alternator in motor mode and to supply the auxiliary systems of the plant. These plants should also include elements of backup power such as batteries or diesel engines to support the buffering of seconds or ensure the normal stop and possibly restart in the event of loss of the grid. Distributed energy storage on the network can help regulate the frequency variations, quickly adjust the energy delivered to the application, to support the production of highly fluctuating power plants using renewable sources and to provide a backup power after a power failure. Moreover, the frequency control is a service designed to reduce the differences in frequencies across networks. The frequency differences result from imbalances between supply and demand for electricity that occur continuously during normal operation or after an incident such as loss of production. The nominal frequency in Europe is set to 50.00 Hz. The minimum frequency is set to Instant 49.2 Hz and the maximum instantaneous frequency is set to 50.8 Hz. This corresponds to a difference of 800 mHz, the maximum permissible frequency deviation from nominal frequency dynamics (ENTSO-E 2009). In practice, the beaches are a wider snapshot of 46 Hz to 52.5 Hz.
There are three levels of frequency control regulation, namely the primary control, secondary control and tertiary control. The generating units of electricity are required to provide a reserve during nominal operation to ensure a primary response frequency control. In Europe, this reserve may vary by country. It is for example +/−2.5% in France and +/−1.5% in Spain.
Activation of the primary reserve is triggered before the frequency deviation from the nominal frequency is greater than 200 mHz and at a minimum time of less than 30 seconds and a maximum of 15 min. The use of energy storage means can therefore also contribute to regulating the frequency in continuous operation with a quick response. Finally, it should regulate the phase shift between voltage and current, resulting in a reactive power control. Charges including windings have a magnetizing effect producing reactive power. This does not work, but summed vectorially with the active power (billing demand), it compiles the power which defines all the energy flowing through the Grid network and system sizing. Optimizing the power factor can reduce Grid network losses, maximize the flow of active power (or the design of smaller facilities) and increase stability. Again, the means of energy storage can be used to regulate this phase shift.
The storage means may also serve as energy sources required when starting plants and to avoid buffering the adverse DC power required for hospitals, data centers and backup systems nuclear power plants.
It is within this context that flywheels are considered as a means of kinetic energy storage. These systems, similar to a battery, consist of the rotation of a flywheel (carbon fibers, other composites, steel, or the like), up to several tens of thousands of revolutions per minute, coupled to a motor/generator. They store/deliver electric energy surplus/deficit on the network, at some point in the form of kinetic energy (Ek) that is recovered by accelerating/decelerating mass. The energy stored/returned is given by the following formula:Ek=½*J*ω2  [EQ 1]
Where J is the moment of inertia (in kg·m2) and ω is the angular velocity (in rad s−1).
To avoid frictional losses, storage systems are supported by magnetic bearings and enclosed in a vacuum chamber. They also include power electronics devices such as a rectifier-inverter combination to ensure the control of the signal injected/removed from the network and, in particular, control of power factor (cos φ). This technology is used, among others, for frequency regulation as a solution for uninterruptible power supply, for power optimization in embedded systems, in areas such as electrical distribution, aerospace, automotive (recovery of kinetic energy during braking), and the railway.
As part of the electrical power distribution and stability of power systems, these storage systems are advantageous because they have a response time less than a second, a life of twenty years, with little maintenance. Moreover, unlike batteries, they have no “memory effect”, they do not fear changes in temperature and charge state is precisely measurable. Finally they do not require recycling, precautions or special operation. Currently and for reference, some of these devices, as marketed now, have a mechanical efficiency of over 95% and a total return (round-trip charge/discharge) of 85%. Some can reach a storage capacity of 25 kWh, delivering an instantaneous power of 250 kW and undergo more than 150,000 cycles of charge/discharge. Various types of systems are known to avoid disruptions of electricity supply and regulate the frequency and power. For example, EP 1900074 and EP 1866717 describe various types of power systems to offset peak demand and prevent service interruptions. EP 1866717 advocates including the use of a mini-network, comprising one or more sources of power generation and one or more independent loads, which can be connected to the grid in response to a disturbance. Documents U.S. 2005 0035744, EP 1656722, EP 359027, U.S. Pat. No. 5,256,907, WO 2002 44 555, U.S. Pat. No. 4,001,666, JP 2003 274562 describe, in turn, the use of flywheels. U.S. 2004 0263116 describes an intelligent system of distribution/storage of energy for power management of the demand side. A device is used for storing electrical energy near the point of use or production. JP 2003 339118 describes a system power supply unit including a distributed wind generation, photovoltaic generation unit, a unit of energy storage, a flywheel and a charging unit.